1. Field of the Invention
The present invention relates to a focus detector, and more particularly, to a focus detector suitable for use in an optical information recording and reproducing apparatus such as an optical disk.
2. Related Background Art
In a prior art optical information recording and reproducing apparatus which records and reproduced information by irradiating a focused light beam to record medium, auto-tracking (AT) of an irradiated light beam is essential in order to attain a high record density. For example, in U.S. Pat. No. 4,079,248, FIG. 1-FIG. 2B, and in the specification thereof at Col. 2, line 19-Col. 4, line 60 show and describe, respectively, a focus detection method for AT.
FIGS. 1A-1C herein illustrate a principle of a prior art focus detector which uses a knife edge. A light beam reflected by a disk 101 as an information record medium is directed to photo-sensing planes 105-106 of a two-split sensor by an objective lens 102 and a sensor lens 103. A knife edge 104 is inserted in a light path of the reflected light beam and is positioned such that light amounts arriving on the photo-sensing planes 105 and 106 are equal in an in-focus state as shown in FIG. 1A. Thus, when the disk is moved away from the infocus position as shown in FIG. 1B, the light amount of the light beam directed to one photo-sensing plane 105 decreases and when the disk moves closer as shown in FIG. 1C, the light amount directed to the other photo-sensing plane 106 decreases. Thus, since the light amounts directed to the photo-sensing planes 105 and 106 of the two-split sensor change depending on the defocus state of the disk, a focusing error signal may be obtained by differentiating the outputs of the respective photo-sensing planes.
However, in the above apparatus, if the disk 101 is inclined by reference numeral .DELTA..theta. as shown by 107 in FIG. 1A by a phone swing due to rotation of the disk, the reflected light 108 shifts as shown by broken lines 109 so that the output of the photo-sensing plane 105 is larger than the output of the photo-sensing plane 106. This state is indistinguishable from the state shown in FIG. 1C where the disk has moved closer and erroneous focus detection takes place. Such error also occurs by an external disturbance such as a shift of light beam on the sensor plane or variation in the light intensity distribution due to other causes such as movement of the objective lens.
On the other hand, focus detectors which operate on principles other than that of the above apparatus are shown and described in U.S. Pat. No. 3,997,715, with respect to FIG. 7 and in the specification thereof at Column 6, line 3-Column 7, line 11, in U.S. Pat. No. 4,654,839, with respect to FIG. 4A-FIG. 4D and in the specification thereof at Column 4, line 52-Column 5, line 65, and in copending U.S. patent application Ser. No. 739,342 now U.S. Pat. No. D295,749 (filed on May 21, 1985 and assigned to the assignee of the present invention), with respect to FIGS. 3-5C and in the specification thereof at page 6, line 7-page 9, line 7. In those apparatus, the focused light beam reflected from a workpiece is split into a peripheral portion and a center portion and the respective split light beams are sensed by separate photo-detectors. A focusing error signal is obtained by a finite difference of the outputs of the photo-detectors.
In those focus detectors, however, if the optical axis is inclined, the proportions of the light amounts of the split light beam change and correct focus detection is difficult to attain.